PRINCIPLES OF ROCK IVEA THERING J I I 



subject here. Rocks, it must be remembered, as the writer has 

 noted elsewhere,' are complex mineral aggregates of low con- 

 ducting power, each individual constituent of which possesses its 

 own ratio of expansion, or contraction, as the case may be. In 

 crystalline rocks these various constituents are practically in 

 contact. In clastic rocks they are, on the other hand, frequently 

 separated from one another by the interposition of a thin layer 

 of calcareous, ferruginous or siliceous matter which serves as a 

 cement. As temperatures rise, each and every constituent 

 expands and crowds with almost resistless force against its 

 neighbor; as temperatures fall, a corresponding contraction 

 takes place. Since in but few regions are surface temperatures 

 constant for any great period of time, it will be readily perceived 

 that almost the world over there must be continuous movement 

 within the superficial portions of the mass of a rock. The actual 

 amount of expansion and contraction of stone under ordinary 

 temperatures, has been a matter of experiment. W. H. Bartlett^ 

 has shown that the average rate of expansion for granite amounts 

 to .000004825 inch, per inch of stone, for each degree Fahrenheit ; 

 for marble .000005668 inch, and for sandstone .000009532 inch. 

 Adie, in a series of similar experiments found the rate of expan- 

 sion for granite to be .00000438 inch, and for white marble 

 .00000613 inch. 3 



Shaler states* that rock surfaces in the eastern United States 

 may be subjected to temperatures varying from 150° F. at 

 midda)' in summer, to 0° and below in winter. This change of 

 150° in a sheet of granite 100 feet in diameter would produce a 

 lateral expansion of 0.8685 inch of surface. That this expan- 

 sion must tend to lessen the cohesion and tear the upper from 

 the deeper lying layers, is self-evident. As exemplifying this. 

 Professor Shaler states that there are on Cape Ann (Massa- 

 chusetts) hundreds of acres of bare rock surface completely 



' Stones for Building and Decoration, Wiley & Sons, New York. 

 = Am. Jour. Science, Vol. XXII, 1832, p. 136. 

 3 Trans. Royal Soc. of Edinburgh, XIII, p. 366. 

 ■♦Proc. Boston Soc. Natural History, XII, 1869, p. 292. 



